The present invention relates to a method of evaluating signals reproduced from an optical disc medium, that stores information by forming therein recording marks of different physical properties from other portions thereof, and to an optical disc drive using the method.
A wide range of optical disc media, including CD-R/RW, DVD-RAM, DVD±R/RW, Blu-ray Disc (or BD) and ones with two data layers, has come into wide use. Commonly used optical disk drives compatible with these discs include a so-called DVD super multi-drive that can record and play CD-R/RW, DVD-RAM and DVD±R/RW discs. Drives with sophisticated functions compatible with BD are expected to become more widespread in the future.
As optical discs gain speed and density, the PRML (Partial Response Maximum Likelihood) reproducing method has become an essential technology. In the PRML method, a decoding of signals reproduced from an optical disc into original binary code strings that were recorded in the disc proceeds by adding new binary codes (bits) one after another to a decoded result obtained so far. When a new bit is added, an assessment is made within a decoder as to whether a bitstream with the new bit added is the most likely bitstream. Based on the result of this assessment, the bit to be added next is determined. So, to obtain correct decoded results by the PRML, i.e., to enhance the quality of the reproduced signals that are finally output from the optical disc to a higher-level apparatus, it is important that the accuracy of the evaluation method be enhanced. As one PRML method an adaptive PRML method or compensation PRML method is available which adaptively changes a target signal level according to the reproduced signal. Non-Patent Document 1 “Tech. Digest ISOM '03, pp. 34” shows that the use of the PRML method allows for a density increase to an equivalent of 35 GB of capacity in a drive capable of handling Blu-ray Disc by compensating for asymmetry of the reproduced signal and for thermal interferences during recording. It is also shown that the reproduction performance under the high-density conditions becomes higher as a restraint length (bit length representing a class) of the PRML method used increases.
Also in optical disc drives using the PRML apparatus, it is important, as with the conventional apparatus, to properly learn (1) a reproduction equalization condition, (2) a focus position and a tilt condition and (3) a recording power and a pulse condition. To this end, various parameters need to be optimized using evaluation indices of reproduced signal quality to make these conditions optimum.
As an example of reproduced signal quality evaluation index that is compatible with the PRML method, a technique to evaluate the quality of a reproduced signal by focusing mainly on an edge shift error is disclosed in Patent Document 1 “JP-A-2003-141823” and Patent Document 2 “JP-A-2005-346897”.
Patent Document 1 “JP-A-2003-141823” discloses a technique that evaluates the quality of a reproduced signal according to a distribution of |Pa-Pb|, where Pa represents a likelihood corresponding to the most likely state change sequence and Pb represents a likelihood corresponding to the second most likely state change sequence.
Patent Document 2 “JP-A-2005-346897” discloses a reproduced signal evaluation technique that uses a pattern having a virtual 1T run length as an error pattern, in which an edge portion of the reproduced signal shifts left and right, and that, based on the direction of edge shift, determines a difference between coded sequence errors to calculate an amount of edge shift.
Disclosed in Patent Document 3 “JP-A-2005-196964”, Patent Document 4 “JP-A-2004-253114” and Patent Document 5 “JP-A-2003-151219” are reproduced signal quality evaluation methods which use a table containing a combination of a correct pattern and an associated erroneous pattern in evaluating the quality of the readout signal. Patent Document 3 and Patent Document 4 disclose a technique that involves calculating differences between a correct pattern Euclidean distance from the reproduced signal and an erroneous pattern Euclidean distance from the reproduced signal, then determining a simulated bit error rate (SbER) from the average of the Euclidean distance differences and a standard deviation, and evaluating the reproduced signal quality based on the SbER. Patent Document 5 discloses a technique to adjust the recording condition so as to make statistical error probability minimal according to the difference between the correct pattern Euclidean distance from the reproduced signal and the erroneous pattern Euclidean distance from the reproduced signal. In a PRML method with a zero target amplitude of a 2T repetition signal, such as a PR(1, 2, 2, 2, 1) method, the frequency of bit errors caused not only by edge shifts but also by 2T shifts and 2T chain collision shifts can no longer be ignored. With Patent Document 3, the quality of reproduced signals can be evaluated by statistically processing the distribution of error probabilities for the correct pattern and erroneous pattern, i.e., for each evaluation bitstream being searched and extracted, and by using an average value and a standard deviation.